Acrylic sheets are made in thicknesses of about 0.08 to about 0.500 inch and used in relatively small areas of about 5 ft..sup.2 and large area pieces of as much as 200 ft..sup.2. They are commonly thermoformed into large consumer and other products such as indoor and outdoor spas, tub/shower enclosures, skylights, bathtubs and highway signs. They may be used as monolithic polymethylmethacrylate sheets or as laminates of such acrylic sheets with other thermoplastics such as ABS. Prior to the present invention it has been known to place a protective film on the acrylic sheet prior to thermoforming, and to employ such film as a protective layer which is intended to be removed only after the thermoforming process. Some kinds of protective materials, such as those based on paper, are totally unsuitable for the thermoforming process since they cannot be stretched or conformed to the shape of a mold--thus, if they are used, they can be used to protect the sheet only up to a point prior to thermoforming, where they must be removed. Frequently new protective wrappings or coatings must be placed on the product after thermoforming. A prior commercial process and product we are familiar with employed a polyethylene film as a protective layer which ostensibly could be left on during thermoforming. But the polyethylene film was often extremely difficult to remove after thermoforming, and caused other problems such as pitting in the acrylic surface due to the generation of air bubbles under the film.
In the case of acrylic/ABS laminates such as are made by the method of U.S. Pat. No. 5,069,851 to Hicks et al and other laminates such as acrylic/PVC laminates, it has been difficult to place the film on the acrylic--a separate step must be included after lamination to do so, since the film will frequently not survive the lamination step.
We have also been made aware of Japanese Patent Application No. 61-257918, filed in Japan Oct. 29, 1986, published May 16, 1988, 63-111050, which describes a film to be left on an acrylic substrate during thermoforming. The film is a two-layer film of low-density polyethylene and ethylene/vinyl acetate which is 20 microns thick. Example 1 of this disclosure describes the application of the film to a two-layer sheet of "MMA/ABS" which is 2.5 mm thick, and the subsequent vacuum molding of it in "an equipment of the male die push-up type". The disclosure does not provide the meaning of "MMA" and does not mention the temperature range, if any heating is used. While it does mention that the "peeling off" of the film is easier after vacuum molding as compared to four other types of film, it does not give their thicknesses and other characteristics.